Photodynamic therapy (PDT), which is the interaction of light with hematoporphyrin derivative (HPD or Photofrin II), is in clinical use in several areas of application. Currently, PDT is being used clinically for bronchial obstructions, carcinoma in situ of the bladder, and intraperitoneal and pleural tumors. Specially designed and constructed fiber-optic probes are modified on a case-by-case basis and calibrated in terms of irradiance delivered to the tissue. Calibrated custom photodiodes are placed at strategic locations within the cavity to provide a record of irradiance values (see project Z01 RR 10272-06). Dosimetry for PDT is complex, due to the variable irradiation levels within the tissue, the sequestering of HPD, and tissue oxygenation.